Cathode for electron discharge devices



Patcnted Jan. 17, 1939 PATENT OFFICE cA'rnom: 'FOR' ELECTRON mscnancs DEVICES Victor 0. Allen, Madison, and Joseph Johnson,

Newark,

N. J., assignors, by mesnc assignments,

- to Radio Corporation of America, New York,

N. Y., a corporation of Delaware N. Drawing. Application October 5, 1935,

Serial No. 43,714

6 Claims. (01. 250-415) Our invention relates to electron discharge devices, more particularly to improvements in thermionic electron emitting cathodes for such devices.

In conventional electron discharge devices with the filament type of thermionic cathode commercial use has been made of pure tungsten filaments,

, activated thoriated tungsten filaments, and oxide coated nickel filaments. The pure tungsten fila- 10 ment cathode is a very stable emitter and is efiicient but must be operated at very high temperatures to obtain ample emission. The activated thoriated tungsten cathode, such as described in U. S. patent to Langmuir 1,244,216, may be operated at lower temperatures than a pure tungsten filamentand is more efficient, but is easily poisoned by oxygen and other gases which adversely affect the emission, is not very stable when operated at high voltages, and requires special processing and seasoning. It has also been found that,

when operated at ultra-high frequencies, for example above 15 or 16 megacycles, emission becomes erratic and there is sometimes a total loss of emission. In addition, the activated thoriated filament is apt to be brittle, and if operated at too high a temperature quickly deactivates and loses itsability to emit a suflicient number of electrons to be useful. The oxide coated nickel cathode operates at a much lower temperature than either kind of tungsten cathode and is eificient, but cannot be satisfactorily operated in tubes in which very high plate voltages, such as 1000 volts, are used, and requires considerable processing and seasoning.

Thorlated molybdenum has also been thoroughly-lnvestigated as a filament cathode emitter, but has proved to be rather unstable, is quite sensi- 'tive to poisoning by gas within the tube, and must be operated at fairly high temperatures, between b.1400 and 1600 C., to provide satisfactory emission. For these and other reasons thoriated molybdenum has not come into commercial use as axcathode filament material.

In the conventional high vacuum or gas tube operating at high'voltages, onlyfllamentary type cathodeshave heretofore been used because the oxide coatings of the indirectly heated cathodes quickly disintegrate at high voltages, and without these coatingsn'io material was available which had sufficient emission unless raised to very high temperatures which cannot be practically generated with the conventional indirectly heated cathode construction.

One object of our invention is to provide a ductile thermionic cathode which is a stable,

efflclent and long lived emitter, at desirable operating temperatures. 'A further object is to provide an efllcient thermionic cathode which can I be satisfactorily used at high frequencies and high voltages and requires little seasoning or process- 5 ing. Our invention is also applicable to indirectly heated cathodes for high voltage gas or vacuum tubes. a

To provide a cathode in accordance with our invention, we have successfully used thoriated l0 molybdenum as a base, upon which chromium has been plated. While this is the preferred embodiment of our invention, it has also been found that, in place of molybdenum, other equivalent refractory metals, suchas tungsten or tantalum, having 15 high melting points, preferably 2400 C. or higher, may be used, as well as equivalents of thoria, such as zirconium, uranium, cerium, titanium, vanadium, yttrium, or lanthanum.

The thoriated molybdenum base'may be pre- 20 pared as a ductile metal in accordance with U. S. Patent 1,082,933 to W. D. Coolidge, or by squirting the finely divided material admixed with a binder. For example, if molybdenum is used we may add powdered nitrate of thoria to the powdered oxide .25 of molybdenum before the reduction of the oxide of molybdenum, or we may add thoria to the oxide of molybdenum after reduction but before consolidation of the metal powder by sintering and mechanical working to the solid state as de- 30 scribed in the said Coolidge patent. We have found in practice that a content of thoria of about 3% by weight of the molybdenum is satisfactory and that a filament of this thoriated metal may be used although the percentage of the thoria 35 may be varied considerably. While thoriatis preferred, equivalents of thoria, for example one of ,the group of metals consisting of zirconium,

uranium, cerium, titanium, vanadium, yttrium and lanthanum may be used with molybdenum to 40 form a base and the base then chromium plated in a conventional chromium plating bath, the essential constituents of which are chromic acid and a sulphate. One example of a satisfactory bath is 33 oz. chromic acid, containing at least 45 95% chromium oxide (CrOa) and not more than 0.2% $04, and .45 grams chemically pure sulphuric acid (H2804) in a gallon of water. The limits of thickness of the chromium plating may be wide, although a very thin plating appears to 50- be entirely satisfactory. While the current density for plating the chromium on the thoriated metal base may vary between wide limits, for example, from a few hundredths of an ampere to over 1 ampere per square centimeter, for best reof emission, to season the filament by operating the electron discharge device containing the filament with normal voltages applied for approximately fifteen minutes in an oscillating circuit.

A cathode made in accordance with our invention has a normal operating temperature several hundred degrees C to below the normal operating temperature of the conventional activated thoriated tungsten cathode, and in the conventional type of tube operated at the customary plate voltage the same plate current can be obtained from our improved cathode at its normal operating temperature with only about one-half the energy per unit area that must be used to obtain the same plate current from the conventional activated thoriated tungsten cathode at its normal operating temperature. For example, with a thoriated molybdenum chromium plated filament made in accordance with our invention having a lighted length of 200 millimeters and measuring mils by 4 mils in cross section and with 9 volts applied to the filament giving a filament current of 2.3 amperes we obtain the same plate current with the same plate voltage applied as with an activated thoriated tungsten filament having a lighted length of 165 millimeters 8 /2 mils in diameter with 10 volts applied to the filament and a flow of 3.25 amperes in the filament. This is equivalent to the dissipation of approximately watts of energy per square centimeter for a thoriated molybdenum chromium plated cathode filament as compared with a dissipation of watts of energy per square centimeter for a thoriated tungsten cathode filament. These tests were made under static conditions with voltages up to 1000 volts on the plate, the tube structures in bothcases being identical except for the cathode filaments.

When the cathodes are to be used in low voltage tubes, for example under 1000 volts, to increase the electron emissivity anyof our cathodes above described may be coated with the usual electron emitting coating of alkaline earth metal oxides, such as barium oxide and strontium oxide, which may be applied in a conventional manner by applying barium and strontium carbonates and converting them into the oxides.

A cathode madein accordance with our invention is a very efficient emitter. It will operate at a comparatively low temperature (1200 C.) and very high voltages up to approximately 3000 volts can be used on the plate of a tube using our cathode without the cathode suffering any ill effects due to ion bombardment. These last two characteristics make our invention particularly suitable for indirectly heated cathodes in high voltage gas or vacuum tubes.

Cathodes made according to our invention are very satisfactory intubes used for generating very high frequencies at high voltagesand while sea soning improves the initial stability and operating characteristics it is not essential in order to provide a cathode which is a good emitter and operates satisfactorily. The resulting cathode is a very stable emitter and is not easily poisoned by gas. The cathode readily recovers its emitting characteristics even if accidentally operated temporarily at too high temperatures. Low voltage ionization will not destroy the tube due to ion bombardment, and the cathode will recover its emission if temporarily impaired by being subjected to high voltages.

While we do not wish to be limited to any particular theory it is believed that the chromium acts as a catalyst to facilitate the rapid diffusion of the electron emitting metal, such as thorium, to the surface of the filament for producing copious emission.

While we have indicated the preferred embodiment of our invention of which we are now aware and have also indicated certain specific applications for which our invention may be employed, it will be apparent that our invention isby no means limited to the exact forms or uses described, but that many variations may be made in the particular structure used, the purpose for which it is employed, and the treatment given to it, without departing from the scope of our invention as set forth in the appended claims.

What we claim as new is:

1. A thermionic cathode for use in an electron discharge device and consisting substantially of molybdenum, in combination with one of the group of metals consisting of thorium, zirconium, uranium, cerium, titanium, vanadium, yttrium and lanthanum and having an electrolytically deposited coating of chromium.

2. A thermionic cathode adapted for use as an electron emitter in electron discharge devices and consisting of substantially of molybdenum, and containing thorium and electrolytically plated with chromium.

3. A thermionic cathode for use in an electron discharge device and consisting substantially of molybdenum containing at least one of the group of metals consisting of thorium, zirconium, uranium, cerium, titanium, vanadium, yttrium and lanthanum and electrolytically plated with chromium.

4. A thermionic cathode for use in an electron discharge device consisting substantially of molybdenum and containing thorium and a coating of chromium electrolytically plated on said metal.

5. A thermionic cathode for use in an electron discharge device'and consisting substantially of molybdenum and containing thorium in a ratio of about 3% by weight and a coating of chromium electrolytically plated on said metal.

6. A thermionic cathode for use in an electron discharge device and consisting substantially of molybdenum containing up to 3% by weight of thorium and a coating of chromium electrolytically plated on the thoriated molybdenum.

VICTOR O. ALLEN. JOSEPH JOHNSON. 

